The proper regulation of gene expression is required for essentially all biological processes. Despite many recent observations that emphasize the importance of mRNA turnover in post-transcriptional gene regulation, little is known about the features of individual mRNAs which influence their decay rate, and virtually nothing is known about the mechanisms of mRNA decay, or the components of the mRNA turnover machinery. The long term goal of our work is to understand the molecular mechanisms which regulate eukaryotic mRNA turnover by exploiting the powerful approaches possible in Saccharomyces cerevisiae. Recent experiments suggest that, at least for some mammalian and yeast mRNAs, there are discrete cis-acting sequences which play a role in determining mRNa decay rate. The goals of this grant are: 1) To define, by a combination of deletion and point mutagenesis, the functional sequences and/or structures within the unstable MAT/alpha1 mRNA which can promote rapid mRNA turnover when transferred to stable reporter mRNAs. 2) To construct genetic screens and selections, exploiting the instability element defined in the above experiments, to identify and characterize mutations in genes encoding components of the mRNA turnover machinery. 3) To identify a pathway of mRNA decay in vivo by overexpressing unstable mRNAs so that we can detect intermediates in the decay pathway. Analysis of the turnover of the MAT/alpha1 mRNA will provide new information about the process of mRNA decay. The genetic strategies possible in yeast provide a unique system for defining the cellular mRNA degradation machinery. Given the fundamental nature of mRNA decay, these studies are highly likely to have significance to more complex eukaryotes.